Portable electronic devices generally require power to operate. The electrical power is often supplied at least partly by one or more batteries. When power in the battery is depleted, a user may replace the battery or recharge it. Recharging the battery often involves physically attaching the battery or the portable electronic device to a power source, such as a standard electrical outlet, via a power cable.
An electrical battery typically includes one or more electrochemical cells that convert stored chemical energy into electrical energy. When the battery is attached to a load (e.g., an electric circuit such as found in a mobile phone), elements within the battery are changed due to a chemical reaction within the battery in order to allow ions (atoms having a positive or negative charge) to move between the battery's positive and negative terminals. This movement of ions permits electrical current to flow out of the battery for use by an electrical device. However, a limited amount of ions is available for moving from one terminal to another, and over time the chemical arrangement within the battery changes such that the ions are depleted. In disposable batteries this chemical rearrangement is permanent. However, in a rechargeable battery, the original chemical arrangement within the battery can be restored by applying a reverse electrical current, thereby restoring ions to a position for generating electrical current. Thus, a battery charger provides the reverse current needed for restoring a battery's charge.
In wired charging devices, the electrical current needed to restore the battery's charge is provided from a wired source, such as a home electrical outlet. For example, a typical charger for an electronic device may include a wall plug, a wire attached at one end to the wall plug, and an electrical connector attached to the other end of the wire. When the wired charger is connected to both the wall outlet and the electronic device, electrical current from the home electrical outlet is supplied through the wire to the electronic device's circuitry, which may directly or indirectly provide the current to a battery attached to the electronic device.
Some electronic devices may include circuitry directed to charging or recharging a battery using wireless power transfer. This technology is especially useful in instances where use of wires may be hazardous, impossible, or simply inconvenient. For example, mobile phones may incorporate wireless power receiving circuitry that can receive power wirelessly from a wireless charging station. Wireless charging stations are available for conveniently charging phones so equipped, reducing the need to attach a power cable to the phone.
In wireless power transfer, power is transferred via electromagnetic fields instead of by the wire used in normal wired power transfer. Such electromagnetic field may operate through air, water, and many other materials. According to electromagnetic principles, a time-varying electrical current in an electrical conductor creates a time-varying magnetic field around the electrical conductor. The magnetic field is strongest near the electrical conductor, and decreases in strength with distance from the electrical conductor. This electromagnetic principle may also operate in reverse. That is, a time-varying magnetic field can induce a time-varying current in an electrical conductor. For example, if an electrically conductive wire passes a permanent magnet, a current is induced in the wire.
In a typical wireless transfer system, a wireless power source provides a time-varying current to an electrically conductive element, often an antenna formed from a coil of wire. That time-varying current generates a time-varying magnetic field around the coil of the power source. A wireless power receiver may also include a conductive element, also typically a coil antenna. If the source coil and receiving coil are sufficiently close together, the time-varying magnetic field around the source coil induces a time-varying electrical current in the receiving coil. This induced electrical current may be used to charge a battery attached to the wireless power receiver.
In a variation of inductive wireless power transfer called “magnetic resonance,” the electrical conductors of the source and receiver are tuned to resonate at a predetermined frequency. A “resonant circuit” permits greater efficiency of energy transfer in many kinds of systems. The reader may be familiar with resonance in an audio setting. In a shower stall, for example, a particular hummed or sung pitch (frequency) may sound louder (higher amplitude) than other pitches hummed or sung with the same energy. The louder pitch is one for which characteristics of the shower stall provide resonance.
Resonance occurs when a system is able to store and easily transfer energy between two or more different storage modes. In the shower example, the shower construction (system) is able to store the hummed/sung pitch (audio energy) in the physical construction of the walls and in the volume of air within the shower. At the resonant frequency this energy is easily transferred between vibrations (oscillations) of the walls and vibrations (oscillations) in the volume of air. Changes in the characteristics (e.g., dimensions or materials) of the shower could change the pitch (frequency) at which the resonance occurs, or could prevent resonance altogether. Because of resonance, the resonant frequency reflected from the shower walls fades much more slowly than non-resonant frequencies
Using the principles of a resonant system, a wireless power transfer system can achieve higher power-transfer efficiency compared to non-resonant inductive charging. This increase in efficiency can permit power transfer over a greater distance compared with normal (non-resonant) inductive charging, and/or may permit faster charging due to comparatively higher amount of power received by a receiving device. The greater distance and efficiency may also permit less precise alignment of source and receiving antennas than is necessary in non-resonant inductive charging systems.
Several standards for wireless power transfer exist or are under development and are competing for dominance in the industry. The Wireless Power Consortium provides the QI™ (pronounced “chee”) standard, and the Power Matters Alliance is developing the PMA™ standard. These proposed standards independently describe primarily inductive power transfer systems and protocols. Both groups are developing magnetic resonance wireless power transfer protocols, although their established bases currently include only inductive power transfer. Alliance for Wireless Power's scheme (A4WP®) presents another competing wireless power transfer scheme focused chiefly on magnetic resonance power transfer at a higher frequency.
In general, in both the inductive and the magnetic resonant power transfer schemes a changing electric current is provided to an electrically conductive source coil. The changing electric current generates a changing magnetic field around the coil, and the strength of the magnetic field corresponds to the strength of the changing electric current in accord with electromagnetic principles. The magnetic field weakens with distance from the source coil. Based on electromagnetic coupling principles, a receiving coil within range reacts to the generated magnetic field, inducing a changing electromotive force (EMF) in the receiving coil. When a load is connected to the receiving coil, the EMF is converted to an electric current that may be used to power a device or charge a battery, for example.
Each of the three main competing wireless power systems includes a data communication channel in addition to the power transfer channel described above. The QI and PMA systems include single-direction communication—from power receiving device to power transmitter—typically using data modulation on the power carrier frequency. For instance, a receiving device may employ “backscatter modulation” in which the amount of power drawn by a perceived load of the receiving device is modulated according to a communication protocol. The receiving coil's change in power draw affects the magnetic coupling between the source coil and receiving coil, and is thus detectable at the power transmitting device. A4WP instead provides two-way wireless data communication between power transmitter and power receive over a separate communications channel (e.g., using BLUETOOTH low energy, a.k.a. BLUETOOTH SMART).
A chief difference between the magnetic resonant and inductive power transfer schemes, as applied in conventional devices, is in the distance between primary and secondary coils permissible for efficient power transfer, and in a corresponding need for precision of alignment needed between the primary and secondary coils. Inductive wireless charging uses low frequency near-field electromagnetic coupling between a primary (or source) antenna and a secondary (or receiving) antenna. Due to limitations of low-frequency, near-field inductive coupling, the distance between the primary and secondary antennas is generally on the order of a few millimeters, and a substantially precise alignment is required between the source and receiving antennas. Magnetic resonance power transfer uses a higher frequency which permits a coil distance of at least four times the diameter of the coil, and need not be so precise in alignment. The power transfer efficiency is substantially lower, however, particularly as distance increases.
Conventional wireless power transfer chargers for portable electronic devices, such as mobile phones, portable meters, media players, tablet computers, notebook computers and the like, have generally been limited to flat-surface formats. That is, the device to be charged is laid on a flat charging platform or mat. In some instances, the charging platform is elevated at an angle and provided with a mating system, such as an array of magnets, to provide a desk-user friendly mount for the device. However, in each instance the flat-surface wireless power transfer charger, particularly in inductive charging implementations, requires the precise placement of the device to be charged, and does not accommodate orientations of the device to be charged other than one in which a main surface of the charging device is substantially parallel to the charger surface.